Track & Field and Athletics: #1 Sports site with latest training info for coaches and self-coached athletesBack to Coachr.org's homepage
or use the search tool on the right

Custom Search

Muscle Fiber Types and Training

By Jason R. Karp, M.S.

How
skeletal muscles adapt to a repeated stimulus depends, to a large extent, on the
inherent characteristics of the muscles themselves. Specifically, the types of
fibers that make up individual muscles greatly influence the way your athletes
will adapt to their training programs. There is a reason why some athletes can
sprint faster and get bigger muscles more easily than others, and why some
athletes are able to run for much longer periods of time without fatigue. In
order to design training pro- grams that will work best for each of your
athletes, it is important for the coach to understand at least some of the
complexity of skeletal muscles.

TYPES OF MUSCLE FIBERS

Humans
have basically three different types of muscle fibers. Slow- twitch (ST or
Type I) fibers are identified by a slow contraction time and a high
resistance to fatigue. Structurally, they have a small motor neuron and fiber
diameter, a high mitochondrial and capillary density, and a high myoglobin
content, Energetically, they have a low supply of creatine phosphate (a
high-energy substrate used for quick, explosive movements), a low glycogen
content, and a wealthy store of triglycerides (the stored form of fat). They
contain few of the enzymes involved in glycolysis, but contain many of the
enzymes involved in the oxidative pathways (Krebs cycle, electron transport
chain). Functionally, ST fibers are used for aerobic activities requiring
low-level force production, such as walking and maintaining posture. Most
activities of daily living use ST fibers.

Fast-twitch
(FT or Type II) fibers are identified by a quick con- traction time and a
low resistance to fatigue. The differences in the speeds of contraction that
gives the fibers their names can be explained, in part, by the rates of release
of calcium by the sarcoplasmic reticulum (the muscle's storage site for calcium)
and by the activity of the enzyme (myosin-ATPase) that breaks down ATP inside
the myosin head of the contractile proteins. Both of these characteristics are
faster and greater in the FT fibers (Fitts & Widrick, 1996; Harigaya &
Schwartz, 1969).

Fast-twitch
fibers are further divided into fast-twitch A (FT -A or Type IIA) and fast-
twitch B (FT -B or Type lIB) fibers. FT -A fibers have a moderate resistance
to fatigue and represent a transition between the two extremes of the ST and FT
-B fibers. Structurally, FT -A fibers have a large motor neuron and fiber
diameter, a high mitochondrial density, a medium capillary density, and a medium
myoglobin content. They are high in creatine phosphate and glycogen and medium
in triglyceride stores. They have both a high glycolytic and oxidative enzyme
activity. Functionally, they are used for prolonged anaerobic activities with a
relatively high force output, such as racing 400 meters.

Fast-twitch
B fibers, on the other hand, are very sensitive to fatigue and are used for
short anaerobic, high force production activities, such as sprinting, hurdling,
jumping, and putting the shot. These fibers are also capable of producing more
power than ST fibers. Like the FT -A fibers, FT -B fibers have a large motor
neuron and fiber diameter, but a low mitochondrial and capillary density and
myoglobin content. They also are high in creatine phosphate and glycogen, but
low in triglycerides. They contain many glycolytic enzymes but few oxidative
enzymes. Table 1 summarizes some major characteristics of the three fiber
types.

Table 1: Characteristics of the Three Muscle Fiber
Types

Fiber Type

Slow Twitch (ST)

Fast Twitch A (FT-A)

Fast Twitch B (FT-B)

Contraction time

Slow

Fast

Very fast

Size of motor neuron

Small

Large

Very large

Resistance to fatigue

High

Intermediate

Low

Activity used for

Aerobic

Long term anaerobic

Short term anaerobic

Force production

Low

High

Very high

Mitochondrial density

High

High

Low

Capillary density

High

Intermediate

Low

Oxidative capacity

High

High

Low

Glycolytic capacity

Low

High

High

Major storage fuel

Triglycerides

CP, Glycogen

CP,
Glycogen

At any given
velocity of movement, the amount of force produced depends on the fiber type.
During a dynamic contraction, when the fiber is either shortening or
lengthening, a fast-twitch (FT) fiber produces more

force than a slow-twitch (ST)
fiber (Fitts & Widrick, 1996). Under isometric conditions, during which the
length of the muscle does not change while it is contracting, ST fibers produce
exactly the same amount of force as FT fibers. The difference in force is only
observed during dynamic contractions. At any given velocity, the force produced
by the muscle increases with the percentage of FT fibers and, conversely, at any
given force output, the velocity increases with the percentage of FT fibers.

There is great
variability in the percentage of fiber types among athletes. For example, it is
well known that endurance athletes have a greater proportion of slow-twitch
fibers, while sprinters and jumpers have more fast-twitch fibers (Costill, et
al., 1976; Ricoy, et al., 1998). The greater percentage of FT fibers in
sprinters enables them to produce greater muscle force and power than their ST
-fibered counterparts (Fitts & Widrick, 1996). Differences in muscle fiber
com- position among athletes have raised the question of whether muscle
structure is an acquired trait or is genetically determined. Studies performed
on identical twins have shown that muscle fiber composition is very much
genetically determined (Komi & Karlsson, 1979), however there is evidence
that both the structure and metabolic capacity of individual muscle fibers can
adapt specifically to different types of training.

RECRUITMENT OF MUSCLE FIBERS

Muscles
produce force by recruiting motor units (a group of muscle fibers innervated by
a motor neuron) along a gradient. During voluntary isometric and concentric
contractions, the orderly pattern of recruitment is controlled by the size of
the motor unit, a condition known as the size principle (Henneman, et al.,
1974). Small motor units, which contain slow-twitch muscle fibers, have the
lowest firing threshold and are recruited first. Demands for larger forces are
met by the recruitment of increasingly larger motor units. The largest motor
units that contain the fast-twitch B fibers have the highest threshold and are
recruited last. No matter what the workout intensity,
slow-twitch motor units are recruited first. If the workout intensity is low,
these motor units may be the only ones that are recruited. If the workout
intensity is high, such as when lifting heavy weights or per- forming intervals
on the track, slow- twitch motor units are recruited first, followed by
fast-twitch A and fast- twitch B, if needed.

There is some
evidence to suggest that the size principle could be altered or even reversed
during certain types of movements-specifically those that contain an eccentric
(muscle lengthening) component-such that fast-twitch motor units are recruited
before slow- twitch motor units (Denier van der Gon, et al., 1985; Grimby &
Hannerz, 1977; Nardone, et al., 1989; Smith, et al., 1980; Ter Haar Romeny, et
al., 1982). It is possible that a preferential recruitment of fast-twitch motor
units, if it exists, is influenced by the speed of the eccentric contraction,
and can only occur using moderate to fast speeds (Karp, 1997; Nardone, et al.,
1989).

DETERMINING FIBER TYPE

Since the only
way to directly determine the fiber-type composition in an athlete is to perform
an invasive muscle biopsy test (in which a needle is stuck into the muscle and a
few fibers are plucked out to be examined under a microscope), some studies have
tried to indirectly estimate the fiber-type composition within muscle groups of
an individual by testing for a relationship between the different properties of
fiber type and muscle fiber composition. This type of research has yielded
promising results, with significant relationships being found between the
proportion of FT fibers and muscular strength or power (Coyle, et al., 1979;
Froese & Houston, 1985; Gerdle, et al., 1988; Gregor, et al., 1979; Suter,
et al., 1993).

An indirect
method that can be used in the weight room to determine the fiber composition of
a muscle

group is to initially establish
the 1RM (the greatest weight that they can lift just once) of your athletes.
Then have them perform as many repetitions at 80% of 1RM as they can. If they do
fewer than seven repetitions, then the muscle group is likely composed of more
than 50% FT fibers. If they can perform 12 or more repetitions, then the muscle
group has more than 50% ST fibers. If the athlete can do between 7 and 12
repetitions, then the muscle group probably has an equal proportion of fibers
(Pipes, 1994).

Because
lifting weights requires the use of many muscles at once, this method does not
work for individual muscles, just muscle groups. In order to determine the
fiber-type composition of an individual muscle, a needle biopsy of the muscle of
interest must be performed.

Another
indirect method that the coach can use, especially when the athletes are young
or new to the sport, is to have the athletes try a number of different events.
Their dominant fiber type will soon become evident based on their success in
certain events, and this discovery can lead to more directed future training for
each athlete.

IMPLICATIONS FOR TRAINING

Your athletes'
fiber type proportion will playa major role in the amount of weight that they
can lift, the number of repetitions that they can complete in a set or interval
workout, and the desired outcome (increased muscular strength/power or
endurance). For example, an athlete with a greater proportion of fast- twitch
fibers will not be able to complete as many repetitions at a given relative
amount of weight as will an athlete with a greater proportion of slow-twitch
fibers and therefore will never attain as high a level of muscular endurance as
will the ST -fibered athlete.

Similarly, an
athlete with a greater proportion of ST fibers will not be able to lift as heavy
a weight or run intervals as fast as will an athlete with a greater proportion
of FT fibers and therefore will never be as strong or powerful as will the FT -
fibered athlete.

It is
important to remember that, even within the group of sprinters or distance
runners on your team, there will still be a disparity in the fiber types. Not
all the sprinters will have the same percentage of FT fibers, nor will all the
distance runners have the same percentage of ST fibers. Therefore, some
sprinters may be able to complete 10x200 meters in a workout while others are
fatigued after 8 repetitions. Likewise, some distance runners may be able to
complete 8x800 meters, while others may fatigue after 5 repetitions.

Depending on
each particular athlete, the coach should decide whether those who fatigue
sooner (because of more FT fibers) should be given longer rest periods between
intervals in order to complete the workout, or should run fewer repetitions at a
faster speed.

Training a FT
-fibered muscle for endurance will not increase the number of ST fibers, nor
will training a ST-fibered muscle for strength and power increase the number of
FT fibers. With the proper training, FT -B fibers can take on some of the
endurance characteristics of FT -A fibers and FT -A fibers can take on some of
the strength and power qualities of FT-B fibers. However, there is no
inter-conversion of fibers. FT fibers cannot become ST fibers, or vice versa.
What an athlete is born with is what he or she must live with.

Although the
type of fiber cannot be changed from one to another , training can change the
amount of area taken up by the fiber type in the muscle. In other words, there
can be a selective hypertrophy of fibers based on the type of training.

For example,
an athlete may have a 50/50 mix of FT/ST fibers in a muscle, but since FT fibers
normally

have a larger cross-sectional
area than ST fibers, 65% of that muscle's area may be FT and 35% may be ST.
Following a strength training program for improvement in muscular strength, the
number of FT and ST fibers will remain the same (still 50/50), however the
cross-sectional area will change. This happens because the ST fibers will
atrophy (get smaller) while the FT fibers will hypertrophy (get larger).

Depending on
the specific intensity used in training, the muscle may change to a 75% FT area
and a 25% ST area. The change in area will lead to greater strength but
decreased en- durance capabilities. In addition, since the mass of FT fibers are
greater than that of ST fibers, the athlete will gain mass, as measured by the
circumference of the muscle.

Conversely, if
the athlete trains for muscular endurance, the FT fibers will atrophy while the
ST fibers hypertrophy, causing a greater area of ST fibers. The area of the
muscle, which began at 65% FT and 35% ST before training, may change to 50% FT
and 50% ST following training, The endurance capabilities of the muscle will
increase while its strength will decrease, and the athlete will lose some muscle
mass, again be- cause ST fibers are lower in mass than FT fibers. The decrease
in mass may be observed by a smaller circumference of the muscle.

Many coaches
know that, for gains in muscular strength, one should train with heavy weights
and few repetitions. This training regimen works because using heavy weights
recruits the FT -B fibers, which are capable of producing a greater force than
the ST or FT -A fibers. Hypertrophy will only occur in those muscle fibers that
are overloaded, so the FT - B fibers must be recruited during training in order
to be hypertrophied (Morehouse & Miller, 1976).

Training with
a low or moderate intensity will not necessitate the recruitment of the FT -B
muscle fibers. Therefore, the training intensity must, be high. But how heavy a
weight and how many repetitions should you use?

Muscular
strength is primarily developed when an 8-repetition maximum (8RM, the maximum
amount of weight that can be lifted eight times ) or less is used in a set. When
the aim of training is to increase the neuro- muscular component of maximum
strength, at least 95% of the athlete's 1RM and 1 to 3 repetitions should be
used. When the aim is to increase maximum strength by stimulating muscle
hypertrophy, at least 80% of 1RM should be lifted 5 to 8 times or until failure
(Zatsiorsky, 1995).

This latter
recommendation assumes that the focus of training is hypertrophy for strength,
rather than hypertrophy simply for muscle size. If the aim of training is to
increase muscle size (hypertrophy) with moderate gains in strength, then 6 to 12
repetitions should be used (Fleck & Kraemer, 1996). Remember, in order to
improve muscular strength, FT -B fibers must be recruited.

For maximum
results, train your athletes according to their genetic predisposition. For
example, an athlete with a greater proportion of slow- twitch fibers would adapt
better to running more weekly mileage and a muscular endurance program, using
more repetitions of a lighter weight. Likewise, an athlete with a greater
proportion of fast-twitch fibers would benefit more from sprint training and a
muscular strength program, using fewer repetitions of a heavier weight.

Jason R. Karp has a master's
degree in exercise physiology and biomechanics. A former university lecturer,
personal trainer, and coach of the Impala Racing Team, he has coached cross
country and track & field at the high school, college, and club levels; A
freelance writer and competitive distance runner who trains all of his muscle
fibers to varying degrees, he is currently pursuing his Ph.D. in exercise
physiology at the University of New Mexico